In the last 30-35 years polymer membrane-based gas separation processes have evolved rapidly. Inorganic membranes were developed during World War II for uranium isotope separation. Both polymer and inorganic membranes, have inherent limitations in terms of one or more of the following desirable membrane properties: selectivity, permeability, and stability. To enhance membrane selectivity and permeability, a new type of membranes, mixed matrix membranes, were developed more recently. To date, almost all of the mixed matrix membranes reported in the literature are hybrid blend membranes comprising insoluble solid domains such as molecular sieves or carbon molecular sieves embedded in a polymer matrix. See U.S. Pat. No. 6,626,980; US 2003/0220188 A1; US 2005/0043167 A1; US 2002/0053284 A1; U.S. Pat. No. 6,755,900; U.S. Pat. No. 6,500,233; U.S. Pat. No. 6,503,295; and U.S. Pat. No. 6,508,860. The strategy for the development of mixed matrix membranes is to incorporate fillers such as solid, liquid, or both solid and liquid fillers into continuous polymer matrices. The polymer matrices are selected from either glassy polymers (e.g., polyimide, polysulfone, polyethersulfone, or cellulose acetate) or rubbery polymers (e.g., silicon rubber).
Ionic liquids (ILs) are a relatively new class of compounds that have received wide attention in recent years as “green” designer solvents that may potentially replace many conventional volatile organic solvents in reaction and separation processes. These unique compounds are organic salts that are liquid over a wide range of temperatures near room temperature and have essentially no measurable vapor pressure. An ionic liquid is a salt in which the ions are poorly coordinated, which results in these solvents being liquid below 100° C., or even at room temperature (room temperature ionic liquids, RTIL's). At least one ion has a delocalized charge and one component is organic, which prevents the formation of a stable crystal lattice.
Recently, ionic liquids have been investigated for their potential for gas separation, including the removal of CO2 from stack gas generated in coal-fired power plants. Most recently, ionic liquids have been used to replace traditional solvents for supported liquid membranes (also called facilitated transport membranes) to take advantage of their unique properties for gas separations. Ionic liquids are particularly attractive in a membrane separation device because their extremely low volatility will minimize solvent losses from the membrane. Supported liquid membranes use porous supports whose pores are impregnated with a solvent such as an ionic liquid. Results for a porous polyethersulfone saturated with ethylmethylimidazolium dicyanamide have shown that the CO2/CH4 ideal selectivity of this supported liquid membrane was above the upper-bound for the CO2/CH4 Robeson plot.
Ionic liquids are particularly attractive in a membrane separation device because their extremely low volatility will minimize solvent losses from the membrane. The durability and retention of the ionic liquids, however, are still not good enough for real process conditions. Ionic liquids must be immobilized in robust, high-flux supports.